The present invention relates generally to odorization of natural gas, and more specifically to an improved system and method for adding odorant to natural gas flowing in a pipeline.
Natural gas is odorless. Because of its potentially dangerous nature, for many years federal regulations have required the addition of an odorant to natural gas so that it can be detected by smell. Odorants such as tertiary butyl mercaptan (TBM) and various blends of commonly accepted chemicals are used in the industry.
The odorants added to natural gas, which are provided in liquid form, are often added to the gas at the location where distribution gas is taken from a main gas pipeline and provided to a distribution pipeline. At this point, the gas pressure is stepped down through a regulator, typically from a pressure of approximately 600 psi or more to a lower pressure of approximately 100 psi or less. The odorants can also be added to gas in the main transmission pipeline.
Odorants used with natural gas are extremely concentrated, so that only a small amount of liquid is needed to odorize a relatively large volume of natural gas. For example, with odorants such as TBM and other blends, it is common to use approximately 0.75 lbs. of liquid odorant to odorize 1,000,000 standard cubic feet (SCF) of natural gas.
Odorants such as TBM and other blends are mildly corrosive, and very noxious. It is important that a correctly measured amount of odorant be added to natural gas; otherwise, various problems will result. For example, over-odorization results in excess odors within the valves, pipes, and other equipment used in natural gas distribution. In addition, too much odorant causes the distinctive odorant smell to be noticeable even after the natural gas is burned. This leads to consumer calls complaining of natural gas leaks, each of which must be responded to by the natural gas distribution company. The expense of such calls, when there is no leak involved, is quite high.
It is also important that the odorant levels not be too low. Safety considerations mandate that a natural gas leak be easily detectable by most people. The proper concentration of odorant within natural gas provides this safety measure, but under-odorization is dangerous because actual leaks may not be detected in time.
One technique in current use to provide odorization to natural gas in a main distribution pipeline involves bypassing a small amount of natural gas, at a slightly higher pressure than the low pressure distribution pipeline, through a tank containing liquid odorant. This bypass gas absorbs relatively high concentrations of the odorant while in the tank. When this heavily odorized bypass gas is placed back into the main pipeline, the odorant, now in gas form, diffuses throughout the pipeline.
Because the bypass gas picks up such large amounts of odorant from the liquid in the tank, becoming completely saturated with odorant gas, it is necessary that carefully monitored small amounts of bypass gas be used. The present approach is to divert the bypass gas through one solenoid valve controlled by a programmable logic controller (PLC). The PLC causes the solenoid valve to open and close for periods of time ranging from fractions of a second to several minutes, depending on the pressure of the bypass gas coming in and the desired pressure coming out. Ideally, the solenoid will close and open fairly often so that the flow of gas is relatively stable. For example, the solenoid valve may be open for 50% of every 10 second period. However, the single-solenoid valve system can be problematic when consumer demand for natural gas changes, such as during seasonal changes. During winter, demand for natural gas is much higher, whereas during summer, the demand is minimal. A single solenoid may have to remain open for very long stretches of time in one season (e.g., ≥90% during winter) and for very short periods of time in another season (e.g., ≤10% during summer), which results in an unstable flow of gas.
It would be desirable to provide an odorization technique, and equipment to be used therewith, which provides both a more reliable and flexible technique for odorizing natural gas. It would be desirable that the equipment be simple and reliable in operation, and that it be flexible enough to easily accommodate significant changes in the operating condition of the gas pipeline and changes in consumer demand for natural gas.